1. Field of the Invention
This invention relates to epitaxially grown films and devices and more particularly to a method of releasing such films and/or devices from the single crystal substrate upon which it is formed to enable its transfer to other substrates and reuse of the single crystal substrate.
2. Description of the Prior Art
In thin film technology there has always been a tradeoff between the material quality of the film and the ease of depositing that thin film. Epitaxial films represent the highest level of quality, but they must be grown on and are accompanied by cumbersome, expensive, bulk single crystal wafer substrates. For some time, research has focused on the possibility of creating epitaxial quality thin films on arbitrary substrates while maintaining the ultimate in crystalline perfection.
The main approach has been to attempt to reuse the substrate wafer by separating it from the epitaxially grown film; however, to undercut a very thin film over its entire area without adversely affecting the film or the underlying substrate, the selectivity must be extremely high. This is very difficult to achieve. For example, J. C. Fan has described a process in which an epitaxial film is cleaved away from the substrate on which it is grown. Such change, at best, is difficult to achieve without damage to the film and/or substrate, or without removal of part of the substrate. Also, in some instances, the cleavage plane (&lt;110&gt;) and the growth plane (&lt;100&gt;) of the film may be mutually exclusive.
In a paper by Konagai et al appearing in J. of Crystal Growth 45, 277-280 (1978) it was shown that a Zn doped p-Ga.sub.1-x Al.sub.x As layer can be selectively etched from GaAs with HF. This observation was employed in the production of thin film solar cells by the following techniques. In one technique, zinc doped p-Ga.sub.1-x Al.sub.x As was grown by liquid phase epitaxy (LPE) on a n-GaAs grown layer on a GaAs single crystal substrate. During this LPE growth of the Zn doped Ga.sub.1-x Al.sub.x As.sub.x, Zn diffuses into the surface of the underlying GaAs to form a p-type GaAs layer and hence p-n GaAs junction. The surface p-Ga.sub.1-x Al.sub.x As is then selectively etched away leaving the p-n junction GaAs layers on the GaAs substrate.
In another solar cell fabrication process Konagai et al describe a "peeled film technology". Here, a 5 micron thick Ga.sub.0.3 Al.sub.0.7 As film is epitaxially grown on a GaAs &lt;111&gt;substrate by LPE. A 30 micron thick Sn doped n-GaAs layer is then grown over the Ga.sub.0.3 Al.sub.0.7 As layer and a p-n junction is formed by diffusing Zn into the specimen utilizing ZnAs.sub.2 as the source of Zn. Appropriate electrical contacts are then formed on the films using known photoresist, etch and plating techniques. The surface layer is then covered with a black wax film supper layer and the wafer is soaked in an aqueous HF etchant solution. The etchants selectively dissolves the the Ga.sub.0.3 Al.sub.0.7 As layer which lies between the thin solar cell p-n junction device layers and the underlying substrate, allowing the solar cell attached to the wax to be peeled off the GaAs substrate for placement on an aluminum substrate. The wax provides support for the peeled film.
While the technique described above has been described in the literature for ten years, it was not adopted by the industry. One reason for this was a difficulty encountered in completely undercutting the Ga.sub.0.3 Al.sub.0.7 As 'release' layer in a reasonable time, especially when the area of the film to be peeled was large. This difficulty arose due to the formation and entrapment of gas, formed as a reaction product of the etching process, within the etched channel. The gas created a bubble in the channel preventing or diminishing further etching and causing cracking in the epitaxial film. The problem could only be partially obviated by using very slow reaction rates (very dilute HF solutions). Since the time required for peel-off, as well as ensuring no or minimal charge to the overlying film is important, the process was virtually abandoned.
A means for providing for the needed circulation of etchant and reaction products and the release of any gaseous reaction products of the etching process while maintaining high selectivity is therefore desired.